This invention relates to imaging systems, and more particularly to methods and systems for detecting components of plaque.
Recent advancements in computed tomography (CT) technology have resulted in faster scanning speeds, larger coverage with multiple detector rows, and thinner slices. However, energy resolution in measurements obtained with these systems is still a missing piece. The wide x-ray photon energy spectrum output from the x-ray source and the lack of energy resolution from CT detection systems preclude energy discrimination CT.
The X-ray attenuation through a given object is strongly dependent on the incident x-ray photon energy. This physical phenomenon manifests itself in an image as beam-hardening artifacts, such as, non-uniformity, shading, and streaks. Some beam-hardening artifacts can be easily corrected, but other beam-hardening artifacts may be more difficult to correct. In general, known methods to correct beam hardening artifacts include water calibration, which includes calibrating each CT machine to remove beam hardening from materials similar to water, and iterative bone correction, wherein bones are separated in a first-pass image reconstruction then the measurements are corrected for the beam hardening resulting from the bones in a second-pass reconstruction. However, beam hardening from materials other than water and bone, such as metals and contrast agents, may be difficult to correct without added processing of the measured data. In addition, even with the above described correction methods, conventional CT does not provide quantitative image values. Rather, the same material at different locations within the image often shows a different CT number.
Another drawback of conventional CT is the reduced level of material characterization. For example, a highly attenuating material with a low density can result in the same CT number in the image as a less attenuating material with a high density. Thus, there is little or no information about the material composition of a scanned object based solely on the CT number.